Abstract: Two new breast cancer treatment protocols, about to be initiated by the National Surgical Adjuvant Project for Breat and Bowel Cancer (NSABP) will be used to, 1. Determine the magnitudeof tamoxifen noncompliance (as measured by self-report, pill counts and blood assays), 2. Determine the sensitivity, specificity and accuracy of each measure of medication compliance relative to the other, 3. Test the hypothesis that tamoxifen noncompliance will increase as a direct function of increases in physiological toxicity, and 4. Test the hypothesis that a preventative intervention can reduce the magnitude of tamoxifen noncompliance. Two studies are planned. In the first study, the magnitudeof tamoxifen noncompliance, as well as physiological (e.g., GI, hematological, hair loss, etc.) and "psychosocial toxicity" (noncompliance, diet changes, anticipatory vomiting, etc.), will be observed int he absence of an intervention. In a second study, an intervention designed to reduce medicaton noncompliance will be implemented and evaluated. The intervention, which will be innitiated by NSABP but sadministered by provider, will consist of educational material, a hot line and an outpatient planning protocol (OPPs) to be administered by cancer clinical nurses. The OPPs will be sued to do a psychosocial needs assessment and to develop individually tailored interventions designed to prevent medication noncompliance.